The present invention relates to a biological information monitor in which a pulse wave propagation time (PWTT) measuring unit and a noninvasive blood pressure (NIBP) measuring unit are used together.
Japanese Patent No. 3,054,084 discloses a related art of a blood pressure monitor apparatus in which a pulse wave propagation time measuring unit and a noninvasive blood pressure measuring unit are used together.
Japanese Patent No. 3,054,084 discloses the following configuration.
Referring to FIG. 3, a blood pressure monitor apparatus 8 includes: a cuff 10 which has a belt-like cloth bag and a rubber bag accommodated in the cloth bag, and which is attached to be wound around, for example, an upper arm 12 of a patient; a pressure sensor 14; a selector valve 16; and an air pump 18. The pressure sensor 14, the selector valve 16, and the air pump 18 are connected to the cuff 10 via a piping 20. The selector valve 16 is configured so as to be selectively switched to one of three states: an inflation state in which pressurized air is allowed to be supplied to the cuff 10; a slow-deflation state in which the pressurized air in the cuff 10 is slowly discharged; and a quick-deflation state in which the pressurized air in the cuff 10 is quickly discharged.
The pressure sensor 14 detects the pressure in the cuff 10, and supplies a pressure signal SP indicative of the detected pressure to each of a static pressure filter circuit 22 and a pulse wave filter circuit 24. The static pressure filter circuit 22 includes a low-pass filter, extracts a static component contained in the pressure signal SP, i.e., a cuff pressure signal SK indicative of the cuff pressure, and supplies the cuff pressure signal SK to an electronic control device 28 via an A/D converter 26. The pulse wave filter circuit 24 includes a band-pass filter, extracts, on a frequency base, a pulse wave signal SM1 which is an oscillating component of the pressure signal SP, and supplies the pulse wave signal SM1 to the electronic control device 28 via an A/D converter 30. The pulse wave signal SM1 represents the cuff pulse wave which is an oscillatory pressure wave that is produced from a brachial artery (not shown) in synchronism with the heartbeat of the patient, and that is then transmitted to the cuff 10.
The electronic control device 28 is configured by a so-called microcomputer including a CPU 29, a ROM 31, a RAM 33, an I/O port (not shown), and the like. The CPU 29 processes signals according to control programs pre-stored in the ROM 31 by utilizing the storage function of the RAM 33, and supplies drive signals through the I/O port to control the selector valve 16 and the air pump 18.
An electrocardiograph 34 continuously detects an electrocardiogram indicative of the action potential of the cardiac muscle through a plurality of electrodes 36 which are applied to predetermined portions of a living body, and supplies a signal SM2 indicative of the electrocardiogram to the electronic control device 28. The electrocardiograph 34 is used for detecting a Q-wave or R-wave of the electrocardiogram which corresponds to a timing when the pumping output of blood from the heart toward the aorta is started.
A photoplethysmogram detecting probe 38 (hereinafter, referred to simply as the “probe”) for a pulse oximeter detects a pulse wave propagated to a peripheral artery including capillaries. For example, the probe is attached to the skin of the living body or the body surface 40 such as a finger tip of the subject in a closely contacted state by a harness (not shown). The probe 38 includes: a container-like housing 42 which opens in one direction; a plurality of first and second light emitting elements 44a, 44b (hereinafter, referred to simply as the light emitting elements 44 in the case where they need not be discriminated from each other) each of which is configured by an LED or the like, and which are disposed on an outer peripheral portion of an inner bottom surface of the housing 42; a light receiving element 46 which is configured by a photodiode, a phototransistor, or the like, and which is disposed on a middle portion of the inner bottom surface of the housing 42; a transparent resin 48 which is integrally disposed in the housing 42 to cover the light emitting elements 44 and the light receiving element 46; and annular shade members 50 which are disposed between the light emitting elements 44 and the light receiving element 46 in the housing 42, for preventing light emitted toward the body surface 40 by the light emitting elements 44 and reflected from the body surface 40, from being received by the light receiving element 46.
For example, the first light emitting element 44a emits red light having a wavelength of about 660 nm, and the second light emitting element 44b emits infrared light having a wavelength of about 800 nm. The first and second light emitting elements 44a, 44b alternately emit the red light and the infrared light for a predetermined time period at a predetermined frequency. The light emitted toward the body surface 40 by the light emitting elements 44 is reflected from a body portion where capillaries densely exist, and the reflected light is received by the common light receiving element 46. The wavelengths of the light emitted from the light emitting elements 44 are not restricted to those described above as far as the first light emitting element 44a emits light having a wavelength which exhibits significantly different absorption factors with respect to oxygenated hemoglobin and reduced hemoglobin, and the second light emitting element 44b emits light having a wavelength which exhibits a substantially same absorption factors with respect to the two kinds of hemoglobin, or namely emit light having a wavelength which is reflected by oxygenated hemoglobin and reduced hemoglobin.
The light receiving element 46 outputs a photoplethysmogram signal SM3 the level of which corresponds to the amount of the received light, through a low-pass filter 52. An amplifier and the like are adequately connected between the light receiving element 46 and the low-pass filter 52. The low-pass filter 52 eliminates noises having a frequency higher than that of the pulse wave, from the photoplethysmogram signal SM3 input thereto, and outputs the resulting signal SM3 from which the noises are eliminated, to a demultiplexer 54. The photoplethysmogram indicated by the photoplethysmogram signal SM3 is a volume pulse wave which is generated in synchronism with the pulse of the patient. The photoplethysmogram corresponds to a pulse-synchronous wave.
The demultiplexer 54 is alternately switched according to a signal from the electronic control device 28, in synchronism with the light emissions of the first and second light emitting elements 44a, 44b, and hence successively supplies, to the I/O port (not shown) of the electronic control device 28, an electric signal SMR due to the red light through a sample-and-hold circuit 56 and an A/D converter 58, and an electric signal SMIR due to the infrared light through a sample-and-hold circuit 60 and an A/D converter 62. When the input electric signals SMR, SMIR are to be output to the A/D converters 58, 62, the sample-and-hold circuits 56, 60 hold the electric signals, respectively, until the operations of converting the electric signals SMR, SMIR which are previously output are completed in the A/D converters 58, 62, respectively.
In the electronic control device 28, the CPU 29 carries out a measuring operation according to programs pre-stored in the ROM 31 by utilizing the storage function of the RAM 33, and outputs a control signal SLV to a drive circuit 64 so as to cause the light emitting elements 44a, 44b to sequentially emit the red light and the infrared light at predetermined frequencies for a predetermined time period. In synchronism with the alternate light emissions by the light emitting elements 44a, 44b, the CPU outputs a switch signal SC to switch over the state of the demultiplexer 54, so that the electric signal SMR is selectively supplied to the sample-and-hold circuit 56, and the electric signal SMIR to the sample-and-hold circuit 60. The CPU 29 calculates the blood oxygen saturation of the living body, based on the amplitudes of the electric signals SMR, SMIR, according to a calculation expression which is previously stored for calculating the blood oxygen saturation.
FIG. 4 is a functional block diagram illustrating the control function of the electronic control device 28 of the blood pressure monitor apparatus 8. Referring to FIG. 4, a blood pressure measuring unit 70 determines the maximal blood pressure value BPSYS, the mean blood pressure value BPMEAN, the minimal blood pressure value BPDIA, and the like, according to an oscillometric method, based on variations of the magnitudes of the pulse wave indicated by the pulse wave signal SM1 which is successively obtained in a slow pressure lowering period in which, after the pressing pressure of the cuff 10 wound around the upper arm of the living body is rapidly increased by a cuff pressure regulating unit 72 to a target pressure value PCM (e.g., a pressure value of about 180 mmHg), the pressure is slowly lowered at a rate of about 3 mmHg/sec.
A pulse wave propagation information calculating unit 74 includes a time-difference calculating unit for, as shown in FIG. 5, successively calculating the time difference (pulse wave propagation time) DTRP from a predetermined portion, for example, the R-wave which is generated in each period of the electrocardiogram successively detected by the electrocardiograph 34, to a predetermined portion, for example, the rising point or lower peak which is generated in each period of the photoplethysmogram successively detected by the probe 38. The pulse wave propagation information calculating unit 74 calculates the propagation velocity VM (m/sec) of the pulse wave propagated through the artery of the subject, according to Expression 1 which is previously stored, based on the time difference DTRP which is successively calculated by the time-difference calculating unit. In Expression 1, L (m) is the distance from the left ventricle to the portion to which the probe 38 is attached, via the aorta, and TPEP (sec) is the pre-ejection period from the R-wave of the electrocardiogram of to the lower peak of the photoplethysmogram. The distance L and the pre-ejection period TPEP are constants, and experimentally obtained in advance.VM=L/(DTRP−TPEP)  (Exp. 1)
A correspondence relationship determining unit 76 previously determines coefficients α and β in a relational expression of the pulse wave propagation time DTRP or the pulse wave propagation velocity VM indicated by Exp. 2 or Exp. 3 and the maximal blood pressure value BPSYS, based on the maximal blood pressure value BPSYS measured by the blood pressure measuring unit 70, and the pulse wave propagation time DTRP or the propagation velocity VM during each blood pressure measurement period, for example, the average value of the pulse wave propagation time DTRP or the propagation velocity VM during the period. Alternatively, a relationship of the mean blood pressure value BPMEAN or minimal blood pressure value BPDIA measured by the blood pressure measuring unit 70 in place of the maximal blood pressure value BPSYS, and the pulse wave propagation time DTRP or the propagation velocity VM in the blood pressure measurement period may be obtained. In short, the selection is made depending upon which one of the maximal, mean, and minimal blood pressure values is selected as a monitor blood pressure value (estimated blood pressure value) EBP.EBP=α(DTRP)+β (where α is a negative constant and β is a positive constant)  (Exp. 2)EBP=α(VM)+β (where α is a positive constant and β is a positive constant)  (Exp. 3)
An estimated blood pressure value determining unit 78 successively determines the estimated blood pressure value EBP based on the actual actual pulse wave propagation time DTRP or propagation velocity VM which is successively calculated by the pulse wave propagation information obtaining unit 74, according to the correspondence relationship (Exp. 2 and Exp. 3) between the blood pressure value of the living body and the pulse wave propagation time DTRP or the propagation velocity VM of the living body.
When the estimated blood pressure value determined by the estimated blood pressure value determining unit 78 exceeds a preset determination reference, the blood pressure measurement by the blood pressure measuring unit 70 is executed.
In a related-art blood pressure monitor apparatus in which a pulse wave propagation time measuring unit and a noninvasive blood pressure measuring unit are used together, the blood pressure measurement by the noninvasive blood pressure measuring unit is executed based on that an estimated blood pressure value determined by an estimated blood pressure value determining unit exceeds a preset determination reference (detection threshold). When, in order to enhance the sensitivity to a change of the estimated blood pressure value, the preset determination reference (detection threshold) is set to be small, the noninvasive blood pressure measuring unit is frequently activated, and a large burden is imposed on the patient, and hence it is difficult to set an adequate detection threshold.